The principles of optical amplification in practical use are based on stimulated emission (or stimulated scattering, parametric amplification). In each case, a gain is obtained by the number of electrons in the upper energy state. The time constant decided by the elementary step of energy supply and the material constant and the structure constant is given to the process where the number of electrons in the upper state by optical amplification is reduced and is recovered with the energy supplied. An erbium doped fiber amplifier (hereafter referred to as “EDFA”), which is the most common optical amplifier, has a time constant of several ms and is therefore supposed to be suitable for the amplification of high speed signals of more than gigabit per second class (as a semiconductor optical amplifier with a short time constant etc. produce a pattern effect, a remedy is necessary for the amplification of high speed signals). However, when the modulation signal is not a series of bit string but a data format with a small granularity like a burst/packet, a transitional excursion occurs in a waveform or an envelope curve even if it is a comparatively long time constant of EDFA (E. Sshulze, M, Malach, F. Raub, “All-Raman amplified links in comparison to EDFA links in case of switched traffic,” in ECOC 2002, Symposium 3.8).
On the other hand, many excursion suppression technologies are reported which are divided roughly into the optical methodology using an optical loop circuit etc. and the electric method using an automatic gain control circuit (AGC) etc. However, the optical methodology is of little practical use due to the complexity of configuration and the low controllability. (C-L. Zhao, H-Y. Tam, B-O. Guan, X. Dong, P. K. A. Wai, X. Dong, “Optical automatic gain control of EDFA using two oscillating lasers in a single feedback loop” Optics communications 225, 157-162 (2003)) On the other hand, the electric method has achieved certain results about the network which changes comparatively at a low speed. However, it has turned out that the electric control system only is not enough as the granularity of a data format becomes smaller and communication methods with a high circuit use efficiency ratio like an optical burst/optical packet have been studied.
According to the examination literature, the time constant of an electric circuit for suppressing a gain excursion is a sub-microsecond, and it cannot respond to the typical length (shortness) of the optical packet which accommodates high-speed payloads of more than 10 gigabits per second class (C. Tian a EDFA pumped by 1480- and 980-nm lasers, “Institute of Electrical and Electronics Engineers JLT 21(8)). 1728-1734 (2003), H. Nakaji, Y. Nakai, M. Shigematsu and M. Nishimura, “Superior high-speed automatic gain controlled erbium-doped fiber amplifiers, “Optical Fiber Technology 9, 25-35 (2003)). Furthermore, an optical controlling method is also proposed. However, as it has various problems, for example, its suppression is imperfect, its method is complication, etc., it has not been put into practical use.
Furthermore, a front formula during the process in which a burst transition of the input signal under WDM environment causes the transient response of EDFA is given approximately (refer to Non-Patent Document 1 below). However, although many methods of adaptively controlling the operating state of EDFA using several external circuits as mentioned above are proposed as a concrete measure to such a transient response, the concrete method of suppressing the transient response of EDFA itself is not provided at all.
FIG. 1 is a figure showing a transition example of a packet waveform brought about by EDFA. FIG. 1 shows an example of a packet which is 9.95328 bits having a 128 bits preamble and a 3814 bits payload, and whose duration is about 400 n second. As shown in FIG. 1, the waveform of an optical packet will collapse by gain excursion of EDFA.
Furthermore, when there is little traffic and if the intensity of all the pulses has been constant, its average power will become smaller. Therefore, the decrease in traffic can be grasped. However, the network control side does not always recognize the labels of each packet (for example, the optical amplification in a transmission channel: 1R relay etc.). The way generally used at the time is the monitoring of an average power. However, only by monitoring an average power, it is impossible to judge whether there is little traffic or the pulse intensity is low when a low average power is detected. In the conventional optical communication system, the EDFA controlled by average power monitor has been operated by APC (power fixed control). Although traffic is heavy and pulse intensity has the same average power as a low signal to such EDFA, if an optical signal with little traffic enters, there arise problems such as a nonlinear phenomenon and a damage of an optical element with very high peak power. On the other hand, if the gain of EDFA is changed, other characteristics (for example, gain flatness) etc. will be influenced. Furthermore, as an optical packet is supposed to go through various paths, the accumulation of the optical loss and optical gain which each packet experience will differ, and the pulse intensity will also differ.
The assumed measures in such a case fall roughly into two methods. One method is to adaptively control the pump optical intensity of EDFA to change the gain. However, this method includes a lot of problems, for example, the response speed is slow as a comparatively large amount of electric current is used, noise figure deteriorates, etc. The other possible method is to drive EDFA by ACC (electric-current fixed control), for example, and at the same time control the incident average power so as to keep the intensity of the optical pulse included therein constant. This method controls the intensity of the optical pulse. Generally, the pulse intensity is decreased. One of the concrete methods is implemented by an optical modulator. As there are many types of optical modulators, the optical pulse can be controlled at high speed by using an optical modulator. In that case, when the incident average power changes greatly (specifically, more than 10 dB), the operating conditions of EDFA will be changed greatly which are optimized to the incident power and gain on a specification with regard to the leaf length or the pump optical intensity. Specifically, the gain flatness of WDM will be lost. When the gain flatness of WDM is lost, WDM has a defect, and the transitional gain excursion changes for every wavelength band. Therefore, a rare earth doped fiber which can equally amplify the intensity of each wavelength channel especially even if there is little traffic etc., and an optical communication system using such a rare earth doped fiber are desired.